1. Field of the Invention
The present invention generally relates to cleaning chemical process equipment. More particularly, it relates to oxidizing iron sulfide in oil refinery equipment.
2. Description of the Related Art including information disclosed under 37 CFR 1.97 and 1.98.
Treating Pyrophoric Material
Spontaneous ignition of iron sulfide either on the ground or inside equipment can occur in all refineries. If this occurs inside equipment like columns, vessels, tanks and exchangers containing residual hydrocarbons and air, the resulting fire and possible explosion can be devastating. Most commonly, pyrophoric iron fires occur during shutdowns when equipment and piping are opened for inspection or maintenance. Instances of fires in crude columns during turnarounds, explosions in storage tanks containing sulfur, crude, or asphalt, vessel over pressurization in, and other safety-threatening events due to pyrophoric iron ignition have been widely reported.
Iron sulfide is one such pyrophoric material that oxidizes exothermically when exposed to air. It can be found in solid iron sulfide scales in refinery units. These iron sulfide scales can be found in the form of pyrite, troilite, marcasite, or pyrrhotite minerals, any of which will react in the presence of oxygen. These scales are formed by the conversion of iron oxide (rust) into iron sulfide in an oxygen-free atmosphere where hydrogen sulfide gas is present. The reaction can be described as:Fe2O3(rust)+3H2S→2FeS+3H2O+S
These conditions commonly exist in closed oil processing equipment made from carbon steel and used to refine high sulfur containing feedstock. The individual crystals of pyrophoric iron sulfides are finely divided, resulting in a very large surface area-to-volume ratio.
When the iron sulfide crystal is subsequently exposed to air, it is oxidized back to iron oxide and either free sulfur or sulfur dioxide gas is formed. This reaction between iron sulfide and oxygen is accompanied by the generation of a considerable amount of heat. This rapid exothermic oxidation is known as pyrophoric oxidation and it can ignite nearby flammable hydrocarbon-air mixtures. The reactions can generally be described by the following chemical equations:4FeS+3O2→2Fe2O3+4S+HEAT4FeS+7O2→2Fe2O3+4SO2+HEAT
The pyrophoric iron sulfide (PIS) remains dormant in the equipment until the equipment is shut down and opened for service, exposing the PIS to air and allowing the exothermic process of rapid oxidation of the sulfides to oxides to occur.
To combat the effects of pyrophoric reactions, the industry has employed several standard procedures:
1. Acid cleaning with a corrosion inhibitor and hydrogen sulfide suppressant. The acid dissolves iron sulfide scale and releases hydrogen sulfide gas. Cleaning/treating with an acid solution can be both effective and inexpensive. There are problems with this approach:                Disposal of the resulting hydrogen sulfide gas can be problematic.        The potential for corrosion can be great when the system contains more than one alloy.        
2. Chelating Solutions
These are specially formulated, high-pH solutions that are effective at dissolving the sulfide deposits without emitting hydrogen sulfide. However, specially formulated chelation solutions for this application are currently costly.
3. Oxidizing Chemicals
Oxidizing chemicals convert the sulfide to oxide. Potassium permanganate (KMnO4) has been used commonly in the past to oxidize pyrophoric sulfide. Potassium permanganate (or sodium permanganate) can be added to the equipment in combination with a water rinse, following a chemical cleaning procedure.
Another problem common to all of the existing methods is related to the nature of the equipment to be treated and the nature of the treatment solution. The pyrophoric material will form on all surfaces where hydrogen sulfide comes in contact with iron oxide. These surfaces can be (and typically are) vertical walls and the underside of horizontal features inside the equipment. Prior methods have used liquid solutions that are difficult (if not impossible) to effectively treat these surfaces without filling the equipment completely with the solution. Much of the equipment in a typical refinery is not designed to support the weight imposed by a complete liquid fill and disposal of the large quantity of residual effluent can be problematic. To address this problem, prior chemistries have been applied using steam to atomize or vaporize them so that once dispersed, they can contact all surfaces of the vessel. The problem with this method of application is that prior chemistries are simple mixtures of various constituents that return to their constituent form when vaporized. Consequently there can be no way to ensure that the proper ingredients are adequately applied.
Reactors present a unique problem with respect to pyrophoric material. Historically, reactors have been flooded with water prior to dumping catalyst. More recently, refiners desire to dry dump the catalyst in their efforts to recover more precious metal and reuse the catalyst. Unfortunately, the process stream frequently contains hydrogen sulfide and nickel carbonyl and possibly other pyrophoric compounds. Upon exposure to air, the catalyst may heat up, igniting residual hydrocarbons in the catalyst.